Selection ends with quantifying the needed thermal dissipation and ensuring the dissipation path. MOSFET selection begins by choosing devices that can handle the required current, given an adequate thermal dissipation path. Moreover, nothing is available to aid power dissipation except a minimal amount of PC-board copper underneath the supply. Thus, the power supply often contends with cramped space, still air, and heat from nearby components. This is especially true for notebook computers, an environment where heatsinks, fans, heatpipes, and other means of disposing heat are typically reserved for the CPU itself. MOSFETs are the most difficult components to specify for high-current power supplies. But because this approach does not create additional board space, it hardly eases the thermal design challenge. A 60A supply, for example, essentially becomes two 30A supplies. Supplies with such high current are typically broken into two or more phases, with each phase handling between 15A and 30A. But while current requirements have increased steadily, the space available for power supplies has not-a fact that has stretched thermal designs to the limit, and beyond. In fact, today's portable core supplies can require up to 60A or more, at between 0.9V and 1.75V. Recently, CPU supply currents have doubled every two years. Perhaps the toughest challenge that designers of portable power supplies face is powering modern high-performance CPUs. It then illustrates these concepts by stepping through the design of one 30A phase of a multiphase, synchronous-rectified, step-down CPU core supply. This article provides step-by-step instructions for calculating the power dissipation of these MOSFETs and determining the temperature at which they operate. Additionally, these MOSFETs are difficult components to specify for notebook products with minimal heat dissipation capabilities. Power MOSFETs are an integral part of any high-power-switching power supplies used in portable devices.
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